1. Field of the Invention
The present invention relates generally to the spray casting of molten metal onto a moving substrate to form a solidified strip of product and, more particularly, relates to a method and apparatus in which a foil is used as the substrate and is continuously moved underneath the spray for receipt of the deposit of metal.
2. BACKGROUND INFORMATION
A process for the production of spray-deposited shaped preforms has been developed by Osprey Metals Ltd. of West Glamorgan, United Kingdom. The Osprey process, as it is generally known, is disclosed in detail in U.K. Pat. Nos. 1,379,261 and 1,472,939 and U.S. Pat. Nos. 3,826,301 and 3,909,921 and in publications entitled "The Osprey Preform Process" by R.W. Evans et al, Powder Metallurgy, Vol. 28, No. 1 (1985), pages 13-20 and ""The Osprey Process for the Production of Spray-Deposited Roll, Disc, Tube and Billet Preforms" by A. G. Leatham et al, Modern Developments in Powder Metallurgy, Vols. 15-17 (1985), pages 157-73.
The Osprey process is essentially a rapid solidification technique for the direct conversion of liquid metal into shaped preforms by means of an integrated gas-atomizing/spray-depositing operation. In the Osprey process, a controlled stream of molten metal is poured into a gas-atomizing device where it is impacted by high-velocity jets of gas, usually nitrogen or argon. The resulting spray of metal particles is directed onto a "collector" where the hot particles re-coalesce to form a highly dense preform. The collector is fixed to a mechanism which is programmed to perform a sequence of movements within the spray, so that the desired preform shape can be generated. The preform can then be further processed, normally by hot-working, to form a semi-finished or finished product.
The spray deposition process has been proposed for producing strip or plate or spray-coated strip or plate, as disclosed in U.S. Pat. Nos. 3,775,156 and 4,779,801 and European Pat. Appln. No. 225,080. For producing these products, a substrate or collector, such as a flat substrate or an endless belt, is moved continuously through the spray to receive a deposit of uniform thickness across its width.
In the case of the production of strip, the substrate onto which the atomized droplets are deposited is an important component of the system. The substrate must function to ensure that the initial droplets of the spray adhere upon splatting to provide a flat bottom surface. In addition, the substrate must also not extract an excessive amount of heat so as to permit sufficient liquid to fill the intersticies between adjacent splats. Porosity will occur when the initial semi-solid deposit layer contains too much solid or is cooled too rapidly by the substrate and provides insufficient liquid to feed the inherent intersticies between the splatted droplets.
In order to produce spray cast strip, the substrate must also be capable of traversing beneath the spray in a continuous manner. Various substrate systems have been proposed including endless belt and rotating wheel configurations. With such systems, several problems arise. Unless the substrate is preheated which may be difficult due to the cooling afforded by the high velocity atomizing gas, too much heat is extracted from the initial splats which promotes porosity in the cast structure. Another problem that may arise is if the substrate becomes excessively hot, sticking may occur whereby the deposits cannot be readily separated from the substrate.
In co-pending U.S. Patent Application, Ser. No. 07/635,090 entitled "Substrate for Spray Cast Strip", in the name of W. G. Watson et al there is disclosed the use of a limited heat-absorptive outer thin foil substrate having limited heat absorption capabilities. However, it has been found that merely using the foil as a substrate is not completely satisfactory. The heat flux from the leading edge of the spray will cause thermal expansion of the foil which may produce wrinkles in the foil or possibly cause the foil to tear. This will result in poor bottom surface characteristics of the cast strip product such as unevenness or excessive porosity.